This invention relates to solid-state imaging devices and more particularly to a lamination type solid-state imaging device in which a photoconductive film is formed on a semiconductor substrate having a charge transfer or switching function and a method of producing such a device.
Conventionally, a solid-state imaging device has been known which has, in combination, a photoconductive film and a circuit element such as a CCD (charge coupled device) having a charge transfer function or an X-Y matrix element having MOS switches arranged in matrix and driven by an X-Y shift register to read signals. Typically, this type of solid-state imaging device comprises, as shown in FIG. 1a, a P-type semiconductor circuit substrate 1, an N.sup.+ -type source region 2a, an N-type drain region 2b, a gate insulating film 3, a gate electrode 4, a first insulating film 5, a second insulating film 6, a metal electrode 7 divided in unit of picture element, a positive hole blocking layer 8, a photoconductive film 9 for conversion of incident light into electric charge, and a transparent electrode 10. The metal electrode 7 constitutes a unit picture element electrode corresponding to a separate picture element and is electrically connected to the N.sup.+ -type source region 2a of the semiconductor substrate 1. The source region 2a also functions to store an electric charge representative of a photo-signal (hereinafter simply referred to as signal charge) created in the photoconductive film 9. Thus, a MOS switch is constituted by the source region 2a, drain region 2b and gate electrode 4. FIG. 1b is a plan view of a lower portion of the semiconductor circuit substrate which underlies the gate electrode 4. Referring to FIG. 1b, electrons are stored in the N.sup.+ -type source region 2a and written into the N-type drain region (transfer region) 2b by the action of the gate electrode 4. The thus written electrons are transferred sequentially by the action of a transfer gate electrode 4a in a longitudinal direction depicted by a dotted arrow. A CCD structure comprised of the transfer region 2b, gate insulating film 3 and gate electrodes 4, 4a participates in the transfer of electrons when suitable clock pulses are applied to the gate electrodes 4 and 4b. FIG. 1a corresponds to a sectional profile (of one picture element) taken on line W--W' in FIG. 1b.
With the conventional device, light l.sub.1 incident on the transparent electrode 10 opposing the metal electrode 7 is absorbed by the photoconductive film 9 having a surface area opposing a surface area of the metal electrode 7 to create electron-positive hole pairs so that electrons are stored in the N.sup.+ -type source region 2a to behave as signal charges. However, light l.sub.2 is not absorbed by the transparent electrode 10 and photoconductive film 9 and it passes through a gap formed in the metal electrode 7 to reach the semiconductor substrate 1, thus creating therein electron-positive hole pairs. While the thus created positive holes penetrate into the substrate 1, the electrons immigrate into the N.sup.+ -type source region 2a for storage of optical information and a charge transfer stage of the CCD or the N-type drain region 2b of the MOS switch. The N-type drain region 2b contributes to transfer of electrons generated in the separate picture element extending in the longitudinal direction. Consequently, the electrons created in the semiconductor circuit substrate 1 and immigrating into the N-type drain region 2b as described previously are spread in the longitudinal direction (transfer direction depicted by dotted arrow), thereby causing a so-called blooming phenomenon whereby spurious signals appear at portions where valid signals are absent.
To prevent such a phenomenon, an expedient has been proposed which is disclosed in Japanese Patent Application Laid-open No. 103934/80. According to this proposal, as shown in FIG. 2, a light shielding member (film) 11 for covering the separation gap formed in a metal electrode 7 is interposed between a first insulating film 5 and a second insulating film 6. In FIG. 2, identical components to those in FIG. 1a are designated by identical numerals and will not be described herein. The structure of FIG. 2 still suffers from the following disadvantages. Firstly, if the light shielding film 11 is not in registry with the gap in the metal electrode 7, the light l.sub.2 reaches the substrate 1. Secondly, if the second insulating film 6 is thick, the light l.sub.2 is liable to leak transversely of the film 6. Especially, a raised portion overlies the substrate near the gate electrodes 4 and 4a as shown in FIG. 2, and the incident light undergoes irregular reflection by the raised portion to aggravate the leakage of light. Conversely, if the second insulating film 6 is thin, there occurs a tendency to short-circuit between the metal electrode 7 and the light shielding member 11. In addition, under the application of high voltage clock pulses to the gate electrodes 4 and 4a, noise tend to occur which, in turn, affect the metal electrode 7 through a stray capacitor between the gate electrode 4 or 4a and the metal electrode 7 to thereby degrade the quality of the images.
Moreover, a highly raised portion overlying the N.sup.+ -type source region 2a disturbs crystal structures in the positive hole blocking layer 8 and photoconductive film 9 to increase the dark current.